Logging tool with response invariant to changes in borehole pressure

Abstract

A logging system for measuring parameters of earth formation penetrated by a well borehole. Measurements made with the system are not adversely affected by varying pressure encountered a borehole environment. This is accomplished by the use of a main compensation element and a detector compensation element to render source and detector geometry invariant to varying pressure. The system is particularly suited for nuclear LWD systems such as back scatter gamma ray density systems. The basic concepts of the system are, however, applicable to other types of nuclear measurement systems that comprise one or more radiation sources, and one or more axially spaced radiation detectors, where system response is a function of source-detector spacing. The basic concepts of the system are also applicable to other types of logging systems, such as electromagnetic and acoustic, where source (transmitter) and sensor (receiver) elements require invariant geometry in order to maximize accuracy of measurements.

1. A tool for operating in a borehole, said tool comprising:
 (a) a tool housing; 
 (b) a source disposed within said tool housing; 
 (c) at least one detector axially spaced from said source and disposed within said tool housing; 
 (d) a detector compensation element cooperating with said at least one detector; and 
 (e) a main compensation element cooperating with said source; wherein 
 (f) said main compensation element and said detector compensation element render axial spacing between said source and said at least one detector invariant to borehole pressure changes. 
 
   
   
     2. The tool of  claim 1  further comprising:
 (a) a source housing in which said source is disposed; and 
 (b) a detector pressure housing containing said at least one detector and said detector compensation element; wherein
 (i) said tool housing comprises a drill collar, 
 (ii) said source housing is rigidly attached to said drill collar, 
 (iii) said detector pressure housing is rigidly attached to said source housing and flexibly attached to said drill collar by means of said main compensation element, and 
 (iv) said at least one detector is flexibly attached to said detector housing by means of said detector compensation element. 
 
 
   
   
     3. The tool of  claim 1  further comprising:
 (a) a source shield in which said source is disposed; 
 (b) a detector shield in which said at least one detector is disposed; and 
 (c) a common pressure housing containing said source shield and said detector shield and said detector compensation element; wherein
 (i) said tool housing comprises a drill collar, 
 (ii) a first end of said common pressure housing is flexibly attached to said drill collar by means of said main compensation element, 
 (iii) a second end of said common pressure housing is rigidly attached to said drill collar, and 
 (iv) said detector shield is flexibly attached to said common pressure housing by means of said detector compensation element. 
 
 
   
   
     4. The tool of  claim 1  further comprising:
 (a) a source shield in which said source is disposed; 
 (b) a detector shield in which said at least one detector is disposed; and 
 (c) a common pressure housing containing said source shield and said detector shield and said detector compensation element; wherein
 (i) said tool housing comprises a drill collar, 
 (ii) a first end of said common pressure housing is rigidly attached to said drill collar, 
 (iii) a second end of said common pressure housing is flexibly attached to said drill collar by means of said main compensation element, and 
 (iv) said detector shield is flexibly attached to said common pressure housing by means of said detector compensation element. 
 
 
   
   
     5. The tool of  claim 1  further comprising:
 (a) a source shield in which said source is disposed; and 
 (b) a detector shield in which said at least one detector is disposed; wherein 
 (c) said tool housing comprises a drill collar; 
 (d) said source shield and said detector shield are disposed within a cavity in the wall of said drill collar; 
 (e) said source shield is flexibly attached to said drill collar by means of said main compensation element; and 
 (f) said detector shield is flexibly attached to said drill collar by means of said detector compensation element. 
 
   
   
     6. The tool of  claim 1  wherein:
 (a) said source emits gamma radiation; 
 (b) two detectors are disposed within said tool housing and at different axial spacings from said source; and 
 (c) said two detectors are responsive to gamma radiation. 
 
   
   
     7. The tool of  claim 6  wherein density of material penetrated by said borehole is determined by combining count rates measured by said two detectors. 
   
   
     8. The tool of  claim 1  wherein said tool is conveyed along said borehole by a drill string. 
   
   
     9. The tool of  claim 1  wherein said main compensation element comprises a spring. 
   
   
     10. A tool for operating in a borehole, said tool comprising:
 (a) a tool housing; 
 (b) a source disposed within said tool housing; 
 (c) at least one detector axially spaced from said source and disposed within said tool housing; 
 (d) a detector compensation element cooperating with said at least one detector, 
 (e) a main compensation element cooperating with said source; 
 (f) a source housing in which said source is disposed; and 
 (g) a detector pressure housing containing said at least one detector and said detector compensation element; wherein
 (i) said tool housing comprises a drill collar, 
 (ii) either said source housing or said detector pressure housing is flexibly attached to said drill collar, and 
 (iii) said main compensation element and said detector compensation element render axial spacing between said source and said at least one detector invariant to borehole pressure changes. 
 
 
   
   
     11. The tool of  claim 10  wherein:
 (a) said source housing is flexibly attached to said drill collar by means of said main compensation element; 
 (b) said detector pressure housing is rigidly attached to said source housing and rigidly attached to said drill collar; and 
 (c) said at least one detector is flexibly attached to said detector pressure housing by means of said detector compensation element. 
 
   
   
     12. The tool of  claim 10  wherein:
 (a) said source housing is rigidly attached to said drill collar; 
 (b) said detector pressure housing is rigidly attached to said source housing and flexibly attached to said drill collar by means of said main compensation element; and 
 (c) said at least one detector is flexibly attached to said detector housing by means of said detector compensation element. 
 
   
   
     13. The tool of  claim 10  further comprising a common pressure housing containing said source shield and said detector shield and said detector compensation element, wherein:
 (a) a first end of said common pressure housing is flexibly attached to said drill collar by means of said main compensation element; 
 (b) a second end of said common pressure housing is rigidly attached to said drill collar; and 
 (c) said detector shield is flexibly attached to said common pressure housing by means of said detector compensation element. 
 
   
   
     14. The tool of  claim 10  further comprising a common pressure housing containing said source shield and said detector shield and said detector compensation element, wherein:
 (a) a first end of said common pressure housing is rigidly attached to said drill collar; 
 (b) a second end of said common pressure housing is flexibly attached to said drill collar by means of said main compensation element; and 
 (c) said detector shield is flexibly attached to said common pressure housing by means of said detector compensation element. 
 
   
   
     15. The tool of  claim 10  wherein:
 (a) said source shield and said detector shield are disposed within a cavity in the wall of said drill collar; 
 (b) said source shield is flexibly attached to said drill collar by means of said main compensation element; and 
 (c) said detector shield is flexibly attached to said drill collar by means of said detector compensation element. 
 
   
   
     16. A method for measuring a parameter in a borehole, said method comprising:
 (a) providing a tool housing; 
 (b) disposing a source within said tool housing; 
 (c) disposing within said tool housing at least one detector axially spaced from said source and; 
 (d) providing a detector compensation element which cooperates with said at least one detector; and 
 (e) providing a main compensation element which cooperates with said source; wherein 
 (f) said main compensation element and said detector compensation element render axial spacing between said source and said at least one detector invariant to borehole pressure changes. 
 
   
   
     17. The method of  claim 16  further comprising:
 (a) disposing said source in a source housing; and 
 (b) disposing said at least one detector and said detector compensation element in a detector pressure housing; wherein
 (i) said tool housing comprises a drill collar, 
 (ii) said source housing is rigidly attached to said drill collar, 
 (iii) said detector pressure housing is rigidly attached to said source housing and flexibly attached to said drill collar by means of said main compensation element, and 
 (iv) said at least one detector is flexibly attached to said detector pressure housing by means of said detector compensation element. 
 
 
   
   
     18. The method of  claim 16  further comprising:
 (a) disposing said source in a source shield; 
 (b) disposing said at least one detector in a detector shield; and 
 (c) disposing said source shield and said detector shield and said detector compensation element in a common pressure housing; wherein
 (i) said tool housing comprises a drill collar, 
 (ii) a first end of said common pressure housing is flexibly attached to said drill collar by means of said main compensation element, 
 (iii) a second end of said common pressure housing is rigidly attached to said drill collar, and 
 (iv) said detector shield is flexibly attached to said common pressure housing by means of said detector compensation element. 
 
 
   
   
     19. The method of  claim 16  further comprising:
 (a) disposing said source in a source shield; 
 (b) disposing said at least one detector in a detector shield; and 
 (c) disposing said source shield and said detector shield and said detector compensation element in a common pressure housing; wherein
 (i) said tool housing comprises a drill collar, 
 (ii) a first end of said common pressure housing is rigidly attached to said drill collar, 
 (iii) a second end of said common pressure housing is flexibly attached to said drill collar by means of said main compensation element, and 
 (iv) said detector shield is flexibly attached to said common pressure housing by means of said detector compensation element. 
 
 
   
   
     20. The method of  claim 16  further comprising:
 (a) disposing said source in a source shield; and 
 (b) disposing said at least one detector in a detector shield; wherein 
 (c) said tool housing comprises a drill collar; 
 (d) said source shield and said detector shield are disposed within a cavity in the wall of said drill collar; 
 (e) said source shield is flexibly attached to said drill collar by means of said main compensation element; and 
 (f) said detector shield is flexibly attached to said drill collar by means of said detector compensation element. 
 
   
   
     21. The method of  claim 16  wherein:
 (a) said source emits gamma radiation; 
 (b) two detectors are disposed within said tool housing; 
 (c) said two detectors are responsive to gamma radiation; and 
 (d) said two detectors are disposed at different axial spacings from said source. 
 
   
   
     22. The method of  claim 21  further comprising combining count rates measured by said two detectors to determine density of material penetrated by said borehole. 
   
   
     23. The method of  claim 16  comprising the additional step of conveying said tool housing along said borehole with a drill string. 
   
   
     24. The method of  claim 16  wherein said main compensation element comprises a spring. 
   
   
     25. A method for measuring a parameter in a borehole, said method comprising:
 (a) providing a tool housing; 
 (b) disposing a source within said tool housing; 
 (c) disposing within said tool housing at least one detector axially spaced from said source; 
 (d) providing a detector compensation element which cooperates with said at least one detector; 
 (e) providing a main compensation element which cooperates with said source; 
 (f) disposing said source in a source housing; and 
 (g) disposing said at least one detector and said detector compensation element in a detector pressure housing; wherein
 (i) said tool housing comprises a drill collar, 
 (ii) either said source housing or said detector pressure housing is flexibly attached to said drill collar, and 
 (iii) said main compensation element and said detector compensation element render axial spacing between said source and said at least one detector invariant to borehole pressure changes. 
 
 
   
   
     26. The method of  claim 25  wherein:
 (a) said source housing is flexibly attached to said drill collar by means of said main compensation element; 
 (b) said detector pressure housing is rigidly attached to said source housing and rigidly attached to said drill collar; and 
 (c) said at least one detector is flexibly attached to said detector pressure housing by means of said detector compensation element. 
 
   
   
     27. The method of  claim 25  wherein:
 (a) said source is rigidly attached to said drill collar; 
 (b) said detector pressure housing is rigidly attached to said source housing and flexibly attached to said drill collar by means of said main compensation element; and 
 (c) said at least one detector is flexibly attached to said detector pressure housing by means of said detector compensation element. 
 
   
   
     28. The method of  claim 25  further comprising:
 (a) disposing said source in a source shield; 
 (b) disposing said at least one detector in a detector shield; and 
 (c) disposing said source shield and said detector shield and said detector compensation element in a common pressure housing; wherein
 (i) a first end of said common pressure housing is flexibly attached to said drill collar by means of said main compensation element, 
 (ii) a second end of said common pressure housing is rigidly attached to said drill collar, and 
 (iii) said detector shield is flexibly attached to said common pressure housing by means of said detector compensation element. 
 
 
   
   
     29. The method of  claim 25  further comprising:
 (a) disposing said source in a source shield; 
 (b) disposing said at least one detector in a detector shield; and 
 (c) disposing said source shield and said detector shield and said detector compensation element in a common pressure housing, wherein
 (i) a first end of said common pressure housing is rigidly attached to said drill collar, 
 (ii) a second end of said common pressure housing is flexibly attached to said drill collar by means of said main compensation element, and 
 (iii) said detector shield is flexibly attached to said common pressure housing by means of said detector compensation element. 
 
 
   
   
     30. The method of  claim 25  further comprising:
 (a) disposing said source in a source shield; and 
 (b) disposing said at least one detector in a detector shield; wherein 
 (c) said source shield and said detector shield are disposed within a cavity in the wall of said drill collar; 
 (d) said source shield is flexibly attached to said drill collar by means of said main compensation element; and 
 (e) said detector shield is flexibly attached to said drill collar by means of said detector compensation element. 
 
   
   
     31. An LWD density logging system for logging a well borehole, the system comprising:
 (a) a drill collar comprising
 (i) a collar wall defined by an inner collar surface and an outer collar surface, and 
 (ii) a cavity within said collar wall and opening at said outer collar surface; 
 
 (b) a source of gamma radiation disposed within a high Z source shield, wherein a source pathway in said high Z source shield extends radially outward from said source thereby forming a source collimator window; 
 (c) a detector pressure housing rigidly disposed within said cavity and rigidly attached to said high Z source shield, the detector pressure housing comprising
 (i) a short spaced gamma ray detector disposed in a first high Z shield and spaced axially at a first distance from said gamma ray source, 
 (ii) a long spaced gamma ray detector disposed in a second high Z shield and spaced axially at a second distance from said gamma ray source, wherein said second distance is greater than said first distance, and wherein said second high Z shield is rigidly attached to said first high Z shield, 
 (iii) a first pathway in said first high Z shield extending radially outward from said short spaced detector thereby forming a short spaced detector collimator window, 
 (iv) a second pathway in said second high Z shield extending radially outward from said long spaced detector thereby forming a long spaced detector collimator window, and 
 (v) a detector compensation element flexibly attaching said second high Z shield to said detector pressure housing; 
 
 (d) a main compensation element that flexibly attaches said high Z source shield to said drill collar; wherein 
 (e) said main compensation element and said detector compensation element render said first distance and said second distance invariant to a variation in the axial length of said detector pressure housing. 
 
   
   
     32. The system of  claim 31  wherein:
 (a) said source of gamma radiation and said high Z source shielding are disposed within a source housing; 
 (b) said source housing is rigidly attached to said detector pressure housing; and 
 (c) said source housing is flexibly attached to said drill collar by means of said main compensation element. 
 
   
   
     33. The system of  claim 31  wherein openings defined by outward bounds of said source pathway and said first pathway and said second pathway are covered with a low Z material. 
   
   
     34. The system of  claim 31  wherein said main compensation element comprises a spring and said detector compensation element comprises a spring. 
   
   
     35. The system of  claim 31  further comprising a processor for combining responses of said short spaced detector and said long spaced detector to obtain a measure of density of material penetrated by said borehole. 
   
   
     36. A method for measuring density of material penetrated by a borehole and while drilling said borehole, the method comprising:
 (a) providing a drill collar comprising
 (i) a collar wall defined by an inner collar surface and an outer collar surface, and 
 (ii) a cavity within said collar wall and opening at said outer collar surface; 
 
 (b) disposing, within said cavity, a source of gamma radiation within a high Z source shield, wherein a source pathway in said high Z source shield extends radially outward from said source thereby forming a source collimator window; 
 (c) rigidly disposing a detector pressure housing within said cavity, wherein said detector pressure housing comprises
 (i) a short spaced gamma ray detector disposed in a first high Z shield and spaced axially at a first distance from said gamma ray source, 
 (ii) a long spaced gamma ray detector disposed in a second high Z shield and spaced axially at a second distance from said gamma ray source, wherein said second distance is greater than said first distance, and wherein said second high Z shield is rigidly attached to said first high Z shield, 
 (iii) a first pathway in said first high Z shield extending radially outward from said short spaced detector thereby forming a short spaced detector collimator window, 
 (iv) a second pathway in said second high Z shield extending radially outward from said long spaced detector thereby forming a long spaced detector collimator window, and 
 (v) a detector compensation element flexibly attaching said second high Z shield to said detector pressure housing; 
 
 (d) rigidly attaching said high Z source shield to said detector pressure housing; and 
 (e) flexibly attaching said high Z source shield to said drill collar by means of a main compensation element; wherein 
 (f) said main compensation element and said detector compensation element render said first distance and said second distance invariant to a variation in the axial length of said detector pressure housing. 
 
   
   
     37. The method of  claim 36  further comprising:
 (a) disposing said source of gamma radiation and said high Z source shielding within a source housing; 
 (b) rigidly attaching said source housing to said detector pressure housing; and 
 (c) flexibly attaching said source housing to said drill collar by means of said main compensation element. 
 
   
   
     38. The method of  claim 36  wherein openings defined by outward bounds of said source pathway and said first pathway and said second pathway are covered with low Z material. 
   
   
     39. The method of  claim 36  wherein said main compensation element comprises a spring and said detector compensation element comprises a spring. 
   
   
     40. The method of  claim 36  further comprising combining responses of said short spaced detector and said long spaced detector in a processor to obtain a measure of density of material penetrated by said borehole. 
   
   
     41. The method of  claim 36  further comprising conveying said drill collar along said borehole with a drill string. 
   
   
     42. An LWD density logging system for logging a well borehole, the system comprising:
 (a) a drill collar comprising
 (i) a collar wall defined by an inner collar surface and an outer collar surface, and 
 (ii) a cavity within said collar wall and opening at said outer collar surface; 
 
 (b) a common pressure housing disposed within said cavity and comprising
 (i) a short spaced gamma ray detector disposed in a first high Z shield and spaced axially at a first distance from a gamma ray source, 
 (ii) a long spaced gamma ray detector disposed in a second high Z shield and spaced axially at a second distance from said gamma ray source, wherein said second distance is greater than said first distance, and wherein said second high Z shield is rigidly attached to said first high Z shield, 
 (iii) a first pathway in said first high Z shield extending radially outward from said short spaced detector thereby forming a short spaced detector collimator window, 
 (iv) a second pathway in said second high Z shield extending radially outward from said long spaced detector thereby forming a long spaced detector collimator window, 
 (v) a detector compensation element flexibly attaching said second high Z shield to said common pressure housing; 
 (vi) a source of gamma radiation disposed within a high Z source shield, wherein a third pathway in said high Z source shield extends radially outward from said source thereby forming a source collimator window, and wherein said high Z source shield is rigidly attached to said first high Z shield; and 
 
 (c) a main compensation element that flexibly attaches one end of said common pressure housing to said drill collar; wherein 
 (d) said main compensation element and said detector compensation element render said first distance and said second distance invariant to a variation in the axial length of said common pressure housing. 
 
   
   
     43. A method for measuring density of material penetrated by a borehole and while drilling said borehole, the method comprising:
 (a) providing a drill collar comprising
 (i) a collar wall defined by an inner collar surface and an outer collar surface, and 
 (ii) a cavity within said collar wall and opening at said outer collar surface; 
 
 (b) disposing a common pressure housing within said cavity, wherein said common pressure housing comprises
 (i) a short spaced gamma ray detector disposed in a first high Z shield and spaced axially at a first distance from a gamma ray source, 
 (ii) a long spaced gamma ray detector disposed in a second high Z shield and spaced axially at a second distance from said gamma ray source, wherein said second distance is greater than said first distance, and wherein said second high Z shield is rigidly attached to said first high Z shield, 
 (iii) a first pathway in said first high Z shield extending radially outward from said short spaced detector thereby forming a short spaced detector collimator window, 
 (iv) a second pathway in said second high Z shield extending radially outward from said long spaced detector thereby forming a long spaced detector collimator window, 
 (v) a detector compensation element flexibly attaching said second high Z shield to said common pressure housing; and 
 (vi) a source of gamma radiation disposed within a high Z source shield, wherein a third pathway in said high Z source shield extends radially outward from said source thereby forming a source collimator window, and wherein said high Z source shield is rigidly attached to said first high Z shield; and 
 
 (c) flexibly attaching one end of said common pressure housing to said drill collar by means of a main compensation element; wherein 
 (d) said main compensation element and said detector compensation element render said first distance and said second distance invariant to a variation in the axial length of said common pressure housing. 
 
   
   
     44. A tool for operating in a borehole, said tool comprising:
 (a) a source; 
 (b) at least one sensor axially spaced from said source and responsive to energy emitted by said source; 
 (c) a detector compensation element cooperating with said at least one sensor; and 
 (d) a main compensation element cooperating with said source; wherein 
 (e) said main compensation element and said detector compensation element render axial spacing between said source and said at least one sensor invariant to borehole pressure changes. 
 
   
   
     45. A method for measuring a parameter in a borehole environs, the method comprising:
 (a) providing a source; 
 (b) axially spacing at least one sensor from said source wherein said at least one sensor yields a response to said source; 
 (c) providing a detector compensation element cooperating with said at least one sensor; 
 (d) providing a main compensation element cooperating with said source; and 
 (e) determining said parameter from said response of said at least one sensor; wherein 
 (f) said main compensation element and said detector compensation element render axial spacing between said source and said at least one sensor invariant to borehole pressure changes.